1. Field of the Invention
The present invention generally relates to lamps and, more particularly, to lamps having a shroud or reflector that has been coated in whole or in part using low pressure chemical vapor deposition (LPCVD).
2. Discussion of the Art
There is an ever present demand for lamps to have a high lumen output. Flat reflectors external to an envelope of a lamp are often used to reflect light energy produced by the lamp and direct the light energy in a desired direction. These reflectors typically have a reflective coating, such as aluminum deposited with an evaporation technique. Aluminum coated reflectors have a reflectance on an average of less than 90% (FIG. 6, curve A), and are prone to degradation caused by external elements. Heat generated from the light source, in the form of infrared light, may also degrade the aluminum coating. In addition, the infrared light is often reflected towards the light producing element, a filament for incandescent lamps or an arc tube for arc lamps, which can shorten the life of the light source. Flat reflectors have less efficiency in directing light output than reflectors having a curved surface to focus light in a desired direction.
Optical interference films which comprise alternating layers of two or more materials of different refractive index have been used to coat reflectors and envelopes for lamps. Such coatings are used to selectively reflect and/or transmit light radiation from various portions of the electromagnetic spectrum such as ultraviolet, visible and infrared radiation. One application in which these coatings have been found to be useful is in the fabrication of dichroic mirrors, also referred to as cold mirrors. A cold mirror in the prior art is a glass or plastic reflector coated on the inside reflecting surface with an optical filter which reflects visible light thereby projecting it forward of the reflector, while at the same time permitting longer wavelength infrared energy to pass through the coating and the reflector. This insures that the light projected forward by the reflector is much cooler than it would otherwise be if both the visible and the infrared light were reflected and projected forward. For example, co-owned U.S. Pat. No. 5,143,445 to Bateman et al. discloses an LPCVD coated cold mirror glass reflector having an optical interference film deposited on both sides of parabolic reflector with an elongated rearward cavity portion.
The present invention provides a shroud for a light producing element. The shroud has an elongated reflecting portion having a curved cross-section and an elongated light-transmissive portion having a curved cross-section. A cavity in which the light producing element is disposed is formed between the reflecting portion and the light-transmissive portion.
According to another aspect of the invention, a lamp has a light transmissive envelope and a light producing element disposed within the envelope. The lamp has a shroud disposed in the envelope and disposed around the light producing element. The shroud has an elongated reflecting portion having a curved cross-section and an elongated light-transmissive portion having a curved cross-section.
According to another aspect of the invention, a method of fabricating a shroud for a light producing element includes the steps of providing an elongated reflecting portion having a curved cross-section and providing an elongated light-transmissive portion having a curved cross-section. The method also includes securing the reflecting portion and the light-transmissive portion together, the light producing element disposed in a cavity formed between the reflecting portion and the light-transmissive portion.